1. Field of the Invention
This invention relates generally to automatic test equipment for electronics (ATE) and, more particularly, to the synthesis of low-noise, high frequency periodic signals for testing microwave and RF circuitry.
2. Description of Related Art
Improvements in high-frequency electronic devices for consumer products such as cellular telephones, pagers, and wireless personal data assistants (PDAs), have given rise to a need for improved electronic testing. At the same time, pressures have been applied to product manufacturers to reduce testing costs.
An important component in the testing of high-frequency electronic devices is the microwave synthesizer. As is known, “synthesizers” are electronic instruments that generate test signals of variable frequency. The test signals are generally single frequency “tones” having low noise. Modern synthesizers include programmable electronics that afford them high frequency resolution over a wide range of frequencies. “Microwave synthesizers” are synthesizers that produce output signals in the microwave frequency band, i.e., in the vicinity of 1 Gigahertz (109) or higher.
A common type of test for a high frequency device involves measuring the electronic noise that the device produces. To perform this type of test, the device under test, or “DUT,” is connected to a test system, or “tester.” The tester generally includes power supplies, a microwave synthesizer, and a sampling instrument. Under control of a test program, the tester activates the power supplies to apply power to the DUT, enables the synthesizer to apply an input signal to the DUT, and enables the sampling instrument to measure an output signal from the DUT. Noise on the output signal is then measured, and measured noise is compared with test limits to determine whether the DUT's noise performance is within the test limits.
For many high-frequency devices, the output signals from the DUT are generally a function of the input signals applied to the DUT. For example, if the input signal has a frequency FIN, the output generally also has the frequency FIN, or a multiple thereof. The exact input-output relationship depends upon the type of device being tested, but some numerical relationship between input and output is almost always present. This being the case, any noise produced by the synthesizer may appear at the output signal. This noise creates an uncertainty in any noise measurement of the DUT, since it is not clear whether the noise being measured is produced by the DUT or injected by the synthesizer.
Therefore, the noise of the synthesizer is a most important specification. By reducing this noise, measurement uncertainties are correspondingly reduced, and the quality of testing is improved.
Because many electronic devices employ some sort of phase modulation scheme, it is particularly critical that synthesizers produce low phase noise. As is known, “phase noise” refers to variations in the phase of signals produced by a device. Phase noise can alternatively be viewed as timing jitter.
Test system developers have sought to develop microwave synthesizers with low phase noise. Their efforts have often entailed developing synthesizers consisting of multiple, adjustable phase-locked loop circuits that operate in unison.
FIG. 1 shows an example of a conventional multi-loop synthesizer 100. The synthesizer 100 shows a synthesizer having three loops; however, it should be understood that multi-loop designs may include a greater or lesser number of loops, as the target application requires.
In the multi-loop synthesizer 100 of FIG. 1, a main phase-locked loop 102 receives a baseband input signal (Baseband In) and produces a microwave output signal (RFOUT). The main phase-locked loop 102 includes a phase comparator 110, a loop filter/amplifier 112, and a VCO (voltage-controlled oscillator) 114, in its forward path, and a series of mixers 116/120 and filters/amplifiers 118/122 in its feedback path.
Additional phase-locked loops 104 and 106 are provided to generate additional high frequency signals. The phase-locked loops 104 and 106 each include a phase comparator 134/154, a loop filter/amplifier 136/156, a VCO 138/158, and a feedback divider 140/160. Input dividers 132/152 are provided to respectively divide an input signal from a clock source 130.
The outputs of the phase-locked loops 104 and 106 are coupled to the mixers 116 and 120 in the main loop 102. The mixers 116 and 120 successively downconvert RFOUT, to produce a much lower frequency feedback signal. The phase detector 110 compares the relatively low frequency feedback signal with Baseband In, and the operation of the loop 102 tends to force the feedback signal to a frequency that equals Baseband In.
To program a desired output frequency, RFOUT, both coarse and fine adjustments are made. The dividers 132, 152, 140, and 160 of loops 104 and 106 are adjusted to establish a coarse output frequency. Baseband In is adjusted, e.g., by programming a direct digital synthesis device (DDS) to tune between the coarse frequency settings made by the dividers.
Many design features of the multi-loop synthesizer 100 promote low phase noise. For example, the clock source 130 is generally a low noise, fixed frequency reference, such as a crystal oscillator. The filters/amplifiers 136 and 156 generally have long time constants, for reducing noise injected into the mixers 116 and 120 of the main loop 102. The main loop 102 is generally free of frequency division, which tends to reduce noise amplification.
The significant benefits of the multi-loop design 100 come at a cost, however. The component count of the circuit is high, and many filters are required. These filters are costly and occupy a large amount of space. In addition, because the multi-loop synthesizer 100 includes multiple feedback circuits that interact, the settling time of the synthesizer 100 is sometimes difficult to predict.
What is needed is a microwave synthesizer that has low phase noise, has predictable settling characteristics, and can be built at lower cost than multi-loop designs.